1. Field of the Invention
The present invention relates to an excimer laser oscillation apparatus used in processing various works, and its driving method.
2. Related Background Art
An excimer laser has received a lot of attention as the one and only high-power laser that can oscillate in the ultraviolet range, and its applications are expected in the electronics, chemical, and energy industries.
More specifically, the excimer laser is used in processes, chemical reactions, and the like of metals, resins, glass, ceramics, photoresists, semiconductors, and the like.
An apparatus that generates an excimer laser beam is known as an excimer laser oscillation apparatus. FIG. 15 shows an example of the arrangement of the apparatus.
A pair of electrodes 54 and 55 are arranged in a laser chamber 51 for storing a laser gas. A voltage application circuit for applying a voltage to excite the laser gas is connected to the pair of electrodes 54 and 55.
A laser gas mixture containing, e.g., F.sub.2, Ar, Kr, Xe, Cl.sub.2, and the like filled in the laser chamber is excited by electron beam radiation, discharging, or the like. The excited atoms bind to atoms in the ground or base bottom state to form molecules that can exist in only an excited state. Such molecules are called excimers. Since the excimers are unstable, they immediately emit ultraviolet rays and drop to the ground state. Such phenomenon is called bond-free transition, and the excimer laser oscillation apparatus amplifies ultraviolet rays obtained by that transition in an optical resonator made up of a pair of reflection mirrors and outputs a laser beam.
Since a KrF laser and ArF laser of excimer lasers use highly reactive fluorine gas as a laser gas, the concentration of fluorine in the laser chamber that stores the laser gas and gives discharging energy to the gas drops eventually. In consideration of this, the voltage supplied to the laser chamber is raised so as to obtain a predetermined output. When the predetermined output becomes hard to obtain even by such control, oscillation is interrupted, and fluorine gas is refilled.
When oscillation continues, the predetermined laser output cannot be obtained, even by refilling fluorine, and the laser chamber must be exchanged in such state.
FIG. 16A shows an example of a conventional laser oscillation circuit. When a thyratron is used as a switch for starting oscillations, after ringing components of a capacitance C.sub.p shown in FIG. 16B are produced. In view of this problem, the thyratron of the laser oscillation circuit is replaced by a thyristor as a semiconductor element to suppress the after ringing components and to prolong the service life of the laser, as shown in FIG. 17A. Note that no light emission occurs at a positive C.sub.p voltage of +5 to +6 kV in FIGS. 16B and 17B since no discharging takes place and no currents flow at that voltage.
However, even with this apparatus, as shown in FIG. 18, fluorine must be refilled at predetermined periods, and oscillations must be done while raising the applied voltage. In other words, fluorine refilling is still required since the fluorine concentration decreases as time passes. For this reason, the service life of the laser chamber is not satisfactory, especially when the laser is used for a long period of time to work articles. The service life of the chamber is an important factor in improving the manufacturing throughput of articles.